Bone loss is a global health concern. The number of people, particularly women, afflicted with bone loss is increasing rapidly as the population over the age of 50 grows. Thus, strategies for detecting bone loss in its early stages are urgently needed.
Healthy bone is in a constant dynamic state of remodeling that is balanced by bone formation and breakdown (Coleman, Cancer Treat. Rev. 27:165-76, 2001). Bone loss occurs when there is a disruption to normal remodeling activity in the form of an increase in the breakdown of bone by osteoclasts without a comparable increase in bone building by osteoblasts. Bone loss can occur as part of the normal aging process (e.g., osteoporosis) or as a complication of cancer, such as bone cancer or bone metastases. Bone metastases can occur by way of the bloodstream when a cancer cell breaks away from a primary tumor and travels in the circulatory system until it becomes lodged in a small capillary network in bone tissue. Cancer cells release various factors that stimulate osteoclastic activity, disrupting bone remodeling balance and causing increased breakdown of bone, such as that which occurs in blood cancers, such as multiple myeloma (MM). Cancers that tend to metastasize to bone include breast, prostate, lung, kidney and thyroid cancers. Unfortunately, many therapies directed to cancer also promote bone loss. For example, hormonal therapies administered in the treatment of breast and prostate cancer can leave subjects more susceptible to bone loss and treatment with chemotherapy leads to bone loss in some subjects.
Osteolytic bone disease is a feature of MM that occurs in approximately 90% of patients during the course of their illness (Pecherstorfer et al., Blood, 90:3743-3750, 1997 and Terpos et al., Leukemia, 24:1700-1712, 2010). Skeletal related events (SREs) defined as 1) pathologic fracture, 2) spinal cord compression, 3) need for palliative radiotherapy, 4) need for orthopedic surgery, and 5) hypercalcemia are a significant source of pain and disability. Myeloma results in an uncoupling of the bone remodeling process, which leads to suppression of osteoblastic activity and upregulation of osteoclastic activity. The pathophysiology of myeloma bone disease involves, among other things, overproduction of the receptor activator of nuclear factor kB ligand (RANKL) by the malignant plasma cell clone. RANKL activates neighboring osteoclasts resident in the bone marrow leading to continuous bone degradation (Roodman, Leukemia 23:435-441, 2009).
Current standard clinical practice for diagnosing bone disease involves the skeletal survey, which comprises a series of plain film x-rays. However, this technique is relatively insensitive in that it requires a large amount of bone damage for positive detection. Further, skeletal survey is of limited use for monitoring a subject's response to treatment. For example, even after a patient has received anti-myeloma and bone-directed therapy, lytic lesions may persist. Other imaging modalities have been studied including CT, MRI, and PET but these techniques are cumbersome, expensive, and the subject of ongoing trials to determine their prognostic utility (Dimopoulos et al., Leukemia 23:1545-1556, 2009).
Bodily fluid-based diagnostic tests are advantageous relative to bone imaging techniques for several reasons including, for example, low invasiveness in sample collection (standard blood draw), low cost, and amenability to high throughput analyses. Such tests are particularly advantageous for monitoring subjects at risk for developing a disease, in part because they allow for collection of baseline data and continued data collection over time.
Dysregulated bone metabolism can be detected in a patient's blood or urine by measuring biochemical resorptive markers such as, for example, N-terminal cross-linked telopeptide (NTx), C-terminal cross-linked telopeptide (CTx), and the ratio of soluble RANKL to osteoprotegrin (sRANKL/OPG).
Currently, the CTx assay is not a standard test used to assess bone loss in MM patients due to high costs. Some studies advocate the use of CTx for MM disease prognosis (Jakob et al., Eur. J. Haematol. 69:37-42, 2002 and Abildgaard et al., Brit. J. Haematol. 120:235-242, 2003). However, larger studies of osteoporosis patients have raised questions regarding the assay's reliability due to a wide range of reference values (age and gender related) when setting up the assay and wide variability of results over a 24 hour period (Baim and Miller, J. Bone Mineral Res. 24:561-574, 2009; Avolio et al., Brazilian Oral Res. 24:250-255, 2010; Fleisher et al., Oral Surg. Oral Pathol. Oral Radiol. Endodontics 110:509-516, 2010; and Lee and Suzuki, Implant Dentistry 18:492-500, 2009).
The principal component of bone is hydroxyapatite, Ca5(PO4)3(OH) and hypercalcemia is a common feature of bone loss and MM. However, a caveat of bone loss assays that quantitate calcium in serum or plasma is that bone loss is likely not the principle source of calcium in the blood. Further, calcium lost from bone would not predominantly be in a ‘soluble’ ion form, which is measured in serum and plasma calcium assays.